1. Field of the Invention
The present invention relates to diamond devices, more particularly to diamond thermistors having high temperature coefficients, and manufacturing methods for the same.
2. Description of the Prior Art
There are two types of thermistors, i.e. PTC (positive temperature coefficient) devices and NTC (negative temperature coefficient) devices. The former are made of barium titanate and the later of silicon carbide for example. The temperature range in which these conventional devices can operate is not so wide and their response speed to temperature change is not so high. A need therefore exists for thermistors to have quick response over a wide temperature renge.
On the other hand, diamond electric devices have recently attracted reseacher's interest. Some attempts have been made to form thermistors by the use of diamond as a thermally sensitive material. The prior art diamond thermistors have small thermistor coefficients and require relatively high voltages to be applied thereacross. The inventor carefully investigated the thermal characteristics of the prior art thermistors. The thermistor coefficients B thereof were measured to be about 7000 (activation energy=0.6 V) when the diamond was not given intentional doping such as boron. The resistance at the contact between the diamond and an electrode, however, was very high. Because of this, it was very difficult to control the distance between electrodes and a relatively high voltage is needed to drive the prior art device and the characteristics of devices were substantially dispersed.
A good ohmic low resistant contact can be obtained when boron is introduced into the diamond. The thermistor coefficient B of the device, however, is lowered to be about 2000 (activation energy=0.21 eV) in case of 300 ppm doping of boron.